Figure out a way to secure the cover/cabinet to the wall, in order to prevent the entire cabinet from falling forward should someone sit or stand upon it to operate the window(s) or blinds etc.
I prefer to make a two-part beveled ledger that will accept the mate by lifting the entire cabinet above and down upon it. Just rip a 1" X 3" board at a 22.5 degree angle. Then attach one piece to the botom of the cover flush to the wall side and the other to the wall. This creates a simple matching pair.
This simple joinery method allows for easy removal of the cabinet/cover by lifting it up and off when it's necessary to bleed the radiator.
Secondly, consider leaving slots or a gap between the wall and backside of the radiator especially if it is located in front of a window.
If you look at the ugly steel covers it seems that there is a reason for their configuration. There is an opening at the bottom and an opening at the top. This allows for convection, and therefore a more rapid exchange of heat from the water/pipe to the air. They make taller covers for drafty rooms -i guess it is for more convection. Unless someone throws out a better idea, I am going to make wood covers that mimic the taller steel design because we have a drafty room to heat.